1. Field of the Invention
The invention relates generally to methods for growing carbon nanotubes and, particularly, to a laser-based method for growing an array of carbon nanotubes.
2. Discussion of Related Art
Carbon nanotubes (CNTs) produced by means of arc discharge between graphite rods were first discovered and reported in an article by Sumio Iijima, entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58). CNTs are electrically conductive along their length, chemically stable, and capable, individually, of having a very small diameter (much less than 100 nanometers) and large aspect ratios (length/diameter). Due to these and other properties, it has been suggested that CNTs can play an important role in various fields, such as microscopic electronics, field emission devices, thermal interface materials, etc.
Generally, there are three conventional methods for manufacturing CNTs. The first method is the arc discharge method, which was first discovered and reported in an article by Sumio Iijima entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58). The second method is the laser ablation method, which was reported in an article, by T. W. Ebbesen et al., entitled “Large-scale Synthesis of Carbon Nanotubes” (Nature, Vol. 358, 1992, pp. 220). The third method is the chemical vapor deposition (CVD) method, which was reported in an article by W. Z. Li entitled “Large-scale Synthesis of Aligned Carbon Nanotubes” (Science, Vol. 274, 1996, pp. 1701). The CVD method is advantageously useful in synthesis of an array of carbon nanotubes and is beneficial in mass production, improved length controllability, compatibility with conventional integrated circuit process, etc.
Generally, mainly three CVD methods, i.e. the thermal CVD, plasma-enhanced CVD, and laser-induced CVD, have been developed for the synthesis of arrays of carbon nanotubes. In conventional laser-induced CVD method, an opaque substrate, such as silicon, is disposed with a catalyst in a closed reactor filled with reactant gases, and either an argon ion laser or CO2 laser is employed to directly irradiate laser light on the substrate to heat the substrate to a reaction temperature. By locally laser-heating the substrate, carbon nanotubes can be synthesized on the substrate.
The conventional laser-induced CVD method for growing carbon nanotubes usually uses solid-state lasers or gas lasers. The solid-state lasers include neodymium-doped yttrium aluminum garnet (Nd: YAG) lasers or argon ion lasers, etc. The gas lasers include CO2 lasers, etc. However, the above-described lasers generally have a large size and high demands on the environment. Further, these lasers generally need an additionally cooling system, a temperature control system, an expensive power supply, a good shockproof system and optical systems, etc. Therefore, these lasers will be restricted to be applied in the laser-induced CVD method for growing carbon nanotubes.
In addition, the above-described laser-induced CVD method is performed in a closed reactor filled with reactant gases. Thus, the above-described method requires a complicated reaction device, and it is difficult to build and/or maintain a huge reactor device for CVD growth of carbon nanotubes on a large area substrate.
Comparing with the above-described solid-state lasers and gas lasers, conventional semiconductor lasers generally have the following advantages. Firstly, the semiconductor lasers only need an ordinary heatsink to disperse heat without an additionally cooling system. Secondly, the semiconductor lasers is simple equipment and can be conveniently used without a temperature control. Thirdly, the semiconductor lasers only require an ordinarily constant current source, and do not need a good shockproof system and optical systems. Therefore, the semiconductor lasers have a smaller size and lower cost than the solid-state lasers and gas lasers. However, the conventional laser-induced CVD method of growing carbon nanotubes do not use semiconductor lasers.
What is needed, therefore, is a laser-induced method for growing an array of carbon nanotubes that use semiconductor lasers.